I just came across this site earlier this year as I was working with a patron. It was a mom who wanted to know if there was a site for books that told parents what the content was (like Kids In Mind) does for movies).
Ta-dah! Facts on Fiction! What I appreciate about the site is that it is not "rating" the books as good or bad (which some of these sites do) but just providing the information and letting the parents make the call.
Another nice non-judgemental site is Common Sense Media, which provides parents with information about movies and TV, but also books, games, music, and web sites.
Anybody have another resource like this that you like to use?